P2- Plasma
Membrane
Structure and Function
Movement of Substances
Key Knowledge
The structure and function of the plasma
membrane and the movement of substances
across it
 The fluid-mosaic model
 The packaging, transport, import and export
of bio macromolecules
 The role of ribosomes, endoplasmic reticulum,
Golgi apparatus and associated vesicles in
the export of proteins
Plasma membrane

This is the insoluble boundary of
the living cell that maintains the
contents of the cell and regulates
the movement of substances in
and out of the cell.

ALL cells have a plasma
membrane.

The plasma membrane is
differentially or selectively
permeable.

The plasma membrane is
composed of two layers of
phospholipids.
Fluid mosaic model
The lipid component of the membrane allows
flexibility and the membrane has the ability to
repair itself.
Glycoprotein
 Sugar
attached to a
protein
 Often
a marker or receptor
molecule that identifies
‘self’ from ‘non self’
 Important
in cell
recognition and
communication
Glycolipid
 Sugar
lipid
 Their
attached to a
role is to provide
energy for the cell and
also as a marker.
Membrane Proteins
Important for:

Regulation of cell behaviour

Organisation of cells in tissues

Cellular communication:
receptor sites detect hormone and other chemical
molecules to
control transmission of messages within and
between cells.
Plasma membrane – the Sieve
 Small
particles can pass through openings or
pores between the phospholipids easily – eg: H20,
CO2, O2 and other small, non polar particles.
 Within the membrane are channels made by
proteins – these allow a variety of larger or polar
substances in and out eg: glucose.
 Large proteins cannot pass through the
membrane.
Plasma membrane – the Sieve
Copy diagram into your workbook
Diffusion

The net or overall movement of particles (molecules or ions)
from a region where they are at a relatively high
concentration to a region where they are at a relatively low
concentration.

Concentration gradient = difference between the two
regions.

Diffusion continues until an equilibrium is reached and there
is the same concentration of particles on either side of the
membrane.

Passive process – no energy is required.

Diffusion is quicker if greater concentration gradient.

http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_diffusion_works.html
Diffusion
What determines the rate of diffusion?
There 4 factors:
1.
The steepness of the concentration gradient. The
bigger the difference between the two sides of the
membrane the quicker the rate of diffusion.
2.
Temperature. Higher temperatures give molecules or
ions more kinetic energy. Molecules move around
faster, so diffusion is faster.
3.
The surface area. The greater the surface area the
faster the diffusion can take place. This is because the
more molecules or ions can cross the membrane at
any one moment.
4.
The type of molecule or ion diffusing. Large molecules
need more energy to get them to move so they tend
to diffuse more slowly. Non-polar molecules diffuse
more easily than polar molecules because they are
soluble in the non polar phospholipid tails.
Osmosis
Terms
 Solute: a substance dissolved into
another substance (the solvent) to
create the solution.
 Solvent:
a substance in which other
substances (solutes) can be dissolved
to create a solution.

E.g. Water is a solvent because it is polar and many other
polar solutes can dissolve in it.
Osmosis
High concentration solution:
High solute concentration and low solvent
concentration.
 Low concentration solution:
Low solute concentration and high solvent
concentration.

Isotonic
Same concentration of solvent and solute
 Hypotonic
Lower concentration (low solute and high
solvent)
 Hypertonic
Higher concentration (high solute and low
solvent)

Osmosis continued...



If a vacuole swells and
pushes the plasma
membrane against the
cell wall it is called
turgid
The pressure on the
cell is called turgor
pressure  keeps cell
firm & maintains
shape.
Opposite of turgid is a
process called
plasmolysis.
Water diffuses out of the
cell and the plasma
membrane is withdrawn
from the cell wall
making the cell limp or
flaccid.
Transport across the membrane
Crenated
Osmosis continued...
 Osmosis
Animation:
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2
/animation__how_osmosis_works.html
 Osmosis: establishing an equilibrium:
 http://www.stolaf.edu/people/giannini/flashanim
at/transport/osmosis.swf
Facilitated Diffusion
 Substances
are helped across the cell membrane
by carrier molecules. Their diffusion is facilitated
(helped).
Facilitated Diffusion
 Transport
proteins in
membrane assist in
movement of simple
sugars, amino acids,
nucleotides, charged
ions and other nutrients.
http://highere.mcgrawhill.com/sites.0072495855/student_view0/chapter3/animation_how_facilitated
_diffusion_works.html
Carrier Proteins
 Assist
the passage of other molecules across the
membrane in facilitated and active transport.
 Carry some molecules to other side of membrane.
Channel Proteins
 Form
pores in membrane that fill with water.
 Lining of protein = hydrophilic
 Particles that are able to dissolve in water move
through membrane this way.
 Selective
 Open only when correct signal received.
Active Transport

Requires input of energy

Cells that undergo lots of active transport of
substances have lots of mitochondria to provide
energy.

Involves a carrier
protein attached
to a source of energy.

Function in one
direction
Bulk Transport
 Large
molecules also need to
move in and out of cells. E.g.
hormones, mucus, enzymes etc.
 Sometimes
this is not as easy as
facilitated diffusion.
 Endocytosis
and exocytosis is
then required.
Endocytosis
Inward movement of particles
into a cell.
The membrane engulfs the
substance and forms a
endocytic vesicle which
stores and transports the
substance throughout the
cytoplasm.


Phagocytosis – encloses solid
material e.g. food

Pinocytosis – encloses
droplets of liquid.
Exocytosis

Movement of particles out
of a cell via the plasma
membrane.

Small membrane bound vesicles move throughout
the cytoplasm to the plasma membrane where it
fuses and releases its contents to the external
environment.

http://www.youtube.com/watch?v=K7yku3sa4Y8
http://highered.mcgrawhill.com/olc.dl.120068.bio02.swf

Synthesis and transport
Involves the coordinated activity of a number of organelles, including
nucleus, ribosomes, endoplasmic reticulum, Golgi apparatus and
vesicles.
STEP 1- mRNA
STEP 7made in nucleus Protein transport vesicle fuses with
enters cytoplasm plasma membrane. Exocytosis:
Protein product secreted
STEP 2- The mRNA
Codes for protein
Being made at
ribosome
STEP 2Polypeptide
produced on
ribosome
STEP2Polypeptide
enters ER tubule
STEP 3Polypeptide modified in ER
tubule environment
STEP 6Protein placed in
protein transport vesicle
STEP 5Further
modification of
polypeptide in
Golgi complex.
Finally ready for
export
STEP 4Polypeptide enclosed in
transport vesicles
Questions
Quiz
 http://www.hbwbiology.net/quizzes/ch8-cell-
membrane.htm
 Complete
the following Questions
Heinemann Biology 2 Textbook:
Chapter 1 Review Question
Key Questions
20-25 page 25
18 – 21 page 44